This paper presents an envelope detector that can be used in an ultra-wide operation frequency range (1 GHz∼10 GHz) and the transient response time of the detector is less than several nanoseconds. The detector comprises an operational trans-conductance amplifier (OTA), a mirror and a buffer. An extra discharging path is added specially to accelerate the falling-down transient process. The value of the discharging current and the choice of the charging/discharging capacitor are the key factors in the transient process. The circuit is implemented using HuaHong 0.18 µm standard CMOS technology. Simulation and measured results are offered showing both better frequency (up to 10 GHz) and transient (several nanoseconds of delay) performance.